The present invention relates to switching-mode power supplies, and especially to an isolated switching power supply having a volt-second clamping circuit.
FIG. 1 illustrates a typical isolated switching power supply. Please refer to FIG. 1. A typical control circuitry for an isolated switching-mode power supply includes a transformer 1, a pulse width modulated (PWM) generator 2, a voltage feedback controller 3, and a metal oxide semiconductor field effect transistor (MOSFET) 4. The transformer 1 has a primary winding electrically connected to voltage supply terminals Vin and a secondary winding for providing an output voltage. The voltage feedback controller 3 is electrically connected to output terminals of the isolated switching power supply and generates a feedback signal with reference to the output voltage of the isolated switching power supply. The feedback signal is fed into the pulse width modulated generator 2 to modulate the duty cycle of the PWM signal thereof. The PWM signal is utilized to control conduction and non-conduction states of the metal oxide semiconductor field effect transistor (MOSFET) 4 to stabilize the output voltage if there are variations at the output voltages due to the load effect or there are variations at the input voltages. However, the duty cycle of the PWM signal in the typical isolated switching power supply is greatly increased so large due to the great increase of the load in the typical isolated switching power supply or the startup condition that the transformer 1 goes into saturation and the metal oxide semiconductor field effect transistor 4 may be destroyed. The typical solution is to increase the primary number of turns or enlarge the transformer core. But this way offers disadvantages including the increase of cost and the weight of the isolated switching power supply.
It is therefore attempted by the applicant to deal with the above situation encountered with the prior art.
It is therefore an object of the present invention to propose an isolated switching power supply having a volt-second clamping circuit to provide a stable output voltage and avoid the saturation of the transformer. Furthermore, according to the present invention, the volume of the transformer in the isolated switching power supply can be reduced and the damage to the primary-side switching element can be avoided.
According to an aspect of the present invention, the control circuitry for an isolated switching-mode power supply includes a transformer having a primary winding electrically connected to voltage supply terminals and a secondary winding for providing an output voltage, a volt-second clamp circuit electrically connected to the primary winding for controlling conduction and non-conduction states of the primary winding so as to achieve a maximum volt-second product limit of an input voltage at the primary winding, and a voltage feedback controller electrically connected to the volt-second clamp circuit for generating a feedback signal with reference to a magnitude of the output voltage and then modulating the volt-second product of the input voltage at the primary winding to stabilize the output voltage.
Preferably, the volt-second clamp circuit includes a sawtooth wave generator for providing a sawtooth wave that increases linearly as a function of time with a slope which is proportional to a supply voltage, a pulse width modulated (PWM) generator for providing a pulse width modulated signal, a diode with its cathode end electrically connected to an output terminal of the pulse width modulated (PWM) generator and its anode end electrically connected to an output terminal of the sawtooth wave generator, a comparator having a noninverting terminal electrically connected to a first voltage level, and an inverting terminal electrically connected to the output terminal of the sawtooth wave generator, a first transistor switch with a control electrode electrically connected to the output terminal of the comparator, a first conduction electrode electrically connected to the output terminal of the pulse width modulated (PWM) generator, and a second conduction electrode electrically connected to ground, a toem-pole circuit with its input terminal electrically connected to the first conduction electrode, and a second transistor switch with a control electrode electrically connected to an output terminal of the toem-pole circuit, a first conduction electrode electrically connected to one terminal of the primary winding, and a second conduction electrode electrically the other terminal of the voltage supply terminals.
Preferably, the first transistor switch is a bipolar junction transistor (BJT).
Preferably, the second transistor switch is a metal oxide semiconductor field effect transistor (MOSFET).
Preferably, the sawtooth wave generator includes a resistor having one end electrically connected to the voltage supply terminal, and a capacitor having one end electrically connected to the other end of the resistor to form a node that is the output terminal of the sawtooth wave generator, and the other end electrically connected to ground.
Preferably, the voltage feedback controller further includes a photo-coupler for providing an isolated protection and feeding the volt-second clamp circuit the feedback signal.
Preferably, the volt-second clamp circuit includes a pulse width modulated (PWM) generator for providing a pulse width modulated signal, a peak holding circuit electrically connected to one end of the secondary winding for obtaining a voltage, a sawtooth wave generator electrically connected to an output terminal of the peak holding circuit for providing a sawtooth wave that increases linearly as a function of time with a slope which is proportional to the supply voltage, a diode with its anode end electrically connected to an output terminal of the sawtooth wave generator, and its cathode end electrically connected to an output terminal of the pulse width modulated generator for being turned on when the pulse width modulated signal is at a low level voltage and turned off when the pulse width modulated signal is at a high level voltage so that the sawtooth wave generator outputs the sawtooth wave when the pulse width modulated signal is at the high level voltage, a comparator having an inverting terminal electrically connected a voltage level, and a noninverting terminal electrically connected to an output terminal of the sawtooth wave generator, a first transistor switch having a control electrode electrically connected to the output terminal of the comparator, a first conduction electrode electrically connected to ground, and a second conduction electrode electrically connected to the output terminal of the pulse width modulated generator, a toem-pole circuit with an input terminal electrically connected the second conduction electrode of the first transistor switch, an isolated transformer having a secondary winding electrically connected to an output terminal of the toem-pole circuit, and a second transistor switch having a control electrode electrically connected to one end of a primary winding of the isolated transformer, a first conduction electrode electrically connected to the other end of the primary winding of the isolated transformer, and a second conduction electrode electrically connected to one end of the primary winding of the transformer.
Preferably, the peak holding circuit includes a first diode having an anode end electrically connected to one end of the secondary winding of the transformer, and a first capacitor having one end electrically connected to a cathode end of the first diode for forming a node which is the output terminal of the peak holding circuit, and the other end electrically connected to ground.
Preferably, the sawtooth wave generator includes a resistor having one end electrically connected to the output terminal of the peak holding circuit, and a capacitor having one end electrically connected to the other end of the resistor to form a node that is the output terminal of the sawtooth wave generator, and the other end electrically connected to ground.
The present invention may best be understood through the following description with reference to the accompanying drawings, in which: